The purpose of this study is to evaluate the efficacy and safety of the combination of Cisplatin,5-Fluorouracil(5FU) and Afatinib as first-line therapy in patients with advanced gastric or gastroesophageal junction cancer. The study will include 55 patients in all. The patients will receive open-label Cisplatin intravenous 75mg/m2 on Day 1, 5FU 750mg/m2 at 24-hour intravenous infusion on Days 1-4, and Afatinib 40mg per os on Days 3-5, 8-12, 15-19.